The present invention relates generally to television signal encoding, and more particularly to the generation of a phase coherent National Television Standards Committee (NTSC) subcarrier.
Specialized combinations of computer hardware and software, such as QuickTime.RTM. by Apple Computer, Inc., allow users to create and edit video movies using a combination of video, graphics, and sound data. Each frame of a movie exists in a digitized baseband (usually red, green, and blue, or RGB) component format, which allows the images to be stored and manipulated by a computer. While the digitized baseband format is ideal for creating and editing movie frames on a computer, this format is not compatible with interlaced television systems or video cassette recorders (VCRs), which require a composite television signal (such as NTSC) as an input.
As is well known to those skilled in the art, the RGB component format can be easily transformed to a basis suitable for NTSC television signal encoding. NTSC composite television signals are generated by combining a baseband luminance (Y) component with two QAM encoded chrominance components (I and Q). Quadrature amplitude modulation encoding, or QAM, is the suppressed carrier amplitude modulation of two subcarriers in quadrature. That is, the baseband I signal is used to modulate an analog sinusoidal subcarrier f.sub.sc, and the baseband Q signal is used to modulate a 90.degree. phase shifted version of f.sub.sc. In the NTSC standard, f.sub.sc is defined to be a 3.579545 MHz (.+-.1 Hz) sinusoidal signal. This, and other signal parameters of the NTSC standard can be found in "Report 624-4, Characteristics of Television Systems," Reports of the CCIR, 1990.
Currently, there are several ways to generate the sinusoidal subcarrier required for QAM encoding of the chrominance components. In the simplest, least accurate systems, f.sub.sc can be generated with an oscillator unlocked to the scan or sample rates of the system. This can lead to difficulties in maintaining the phase coherency of the subcarrier. Other, more complex systems incorporate phase locked loop circuitry to more accurately generate a phase coherent subcarrier. In yet another approach, a digital frequency synthesizer is used to generate a sequence of samples that may be used to derive f.sub.sc.
While each of these systems works for its intended purpose, the systems capable of generating a highly accurate phase coherent subcarrier are expensive, and can be difficult to implement in a system where the subcarrier frequency is not conveniently related to the video sampling rate or one of its harmonics. For example, the duration of a horizontal scan line is defined to be (in the NTSC standard) 63.5555 .mu.sec, 10.7 .mu.sec of which is used for horizontal blanking. Thus, 52.8555 .mu.sec per scan line contains picture information. In a system having 640 pixels per scan line, each pixel is presented for (52.8555 .mu.sec per line/640 pixels per line)=82.586717 nsec per pixel. This implies that a sample rate of (1/82.586717 nsec)=12.108484 MHz must be used to accurately sample each pixel.
As can be shown with simple division, the relationship between this sample rate and f.sub.sc is not a simple one: (12.108484 MHz/3.579545 MHz)=3.382688. Thus, deriving f.sub.sc by using the horizontal sampling frequency as a reference is not practical.
Accordingly, an object of the present invention is to simply and efficiently generate a phase coherent NTSC subcarrier by using commonly available digital video processing signals as a reference, when such signals do not have a simple relationship to the NTSC subcarrier frequency.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.